1. Field of the Invention
The present invention relates to a voltage reference generator, and more particularly to a voltage reference generator that generates a read voltage reference applicable to the cells of an EEPROM memory.
2. Discussion of the Related Art
The read voltage applied to the cells of a memory circuit is usually produced from the logic supply voltage of the circuit. It typically has a level below this supply voltage, for example, on the order of 2.5 volts. This read voltage should be stable to enable reliable reading of the memory cells, meaning that it should not vary with the supply voltage or the temperature. It should also be reproducible from one circuit to another despite the fact that the characteristics, for example the threshold voltage of the transistors, may vary dependent on the particular manufacturing method used.
The circuits used to generate a voltage reference of this kind generally use a current mirror structure with means to compensate for the different variations of temperature, supply voltage, etc. An example of such a structure is given for example in the French patent application No. FR 95 09023. Other circuits use a differential amplifier mounted as a follower amplifier to deliver, at the output, the reference voltage given by a voltage source at its positive input.
Usually, the read voltage reference generation circuit associated with a memory in an integrated circuit works continuously, once the integrated circuit is no longer in standby mode. However, it is a design goal to limit the consumption of the integrated circuits.
One way of limiting the consumption in an integrated circuit of this kind is to activate the read reference circuit only when it is needed, namely on demand, when there actually is a read operation to be carried out. However, using conventional read reference circuits, a problem arises. Because the read voltage reference is applied to a highly capacitive line, the build-up time of this read voltage is very long. This problem is further aggravated when these circuits are based on current mirror structures which require a long time to reach the state of equilibrium. Accordingly, conventional voltage reference circuits cannot be used on demand.